The present invention relates to an AM/FM receiver, and particularly to an automatic tuning system used in an AM/FM receiver of a digital tuning type.
In recent years, an AM/FM receiver has been equipped with an automatic tuning system to receive automatically AM and FM broadcasting stations. Such an automatic tuning system comprises a frequency scanning section for scanning received frequencies, a station detector section for detecting whether or not an AM or FM broadcasting station is present at the received frequency, and a scanning stop section for stopping the operation of the frequency scanning section in response to a detection signal from the station detector section. As a result, the AM/FM receiver is tuned to a present AM or FM broadcasting station without turning a tuning knob provided on the AM/FM receiver set.
In prior art, the station detector section has been composed of an amplifier having a very narrow band-pass characteristic, a rectifier, and a level comparator. The amplifier responds only to a signal having a predetermined intermediate frequency by its very narrow band-pass characteristic and amplifies the signal. The rectifier rectifies the signal produced by the amplifier to convert it into a D.C. level. The D.C. level is compared with a reference level by the level. comparator, so that the comparator produces the detection signal for stopping the frequency scanning operation when the D.C. level exceeds the reference level Accordingly, the scanning operation of the frequency scanning section is stopped only when an AM or FM broadcasting station is received with a desired electrical field strength, and the AM/FM receiver is turned to the AM or FM broadcasting station.
However, in order to realize a desired band-pass characteristic in the amplifier, a plurality of circuit elements which cannot be formed in an integrated circuit device are required. In other words, the station detector section according to the prior art is not suitable for an integrated circuit device. Moreover, the band-pass characteristic of the amplifier fluctuates from a desired value due to the change in temperature and/or the deviation of the respective circuit elements. For this reason, it often occurs that the station detector section does not detect the presence of AM and/or FM broadcasting stations.